Researchers unravel the mystery of the Arrow of Time

New York, August 29, 2022 – The passage of time from past to future is a central feature of our experience of the world. But precisely how this phenomenon, known as the arrow of time, arises from microscopic interactions between particles and cells is a mystery that researchers at the CUNY Graduate Center Initiative for the Theoretical Sciences (ITS) are helping to solve with the publication of a new article in the journal Physical Review Letters. The results could have important implications in various disciplines, including physics, neuroscience and biology.

Basically, the arrow of time stems from the second law of thermodynamics: the principle that the microscopic arrangements of physical systems tend to increase randomly from order to disorder. The more disordered a system becomes, the more difficult it is for it to find its way back to an ordered state, and the stronger the arrow of time. In short, the universe’s tendency toward disorder is the fundamental reason we experience time flowing in one direction.

“The two questions our team asked were: if we looked at a particular system, would we be able to quantify the strength of its arrow of time, and would we be able to determine how it emerges from the microscale, where cells and neurons interact, to the whole system?” said Christopher Lynn, first author of the paper and postdoctoral fellow in the ITS program. “Our findings are the first step in understanding how the arrow of time that we live in the everyday life emerges from these more microscopic details.”

To begin to answer these questions, the researchers explored how the arrow of time could be broken down by looking at specific parts of a system and the interactions between them. The parts, for example, could be the neurons that function in a retina. Looking at a single moment, they showed that the arrow of time can be broken down into different pieces: those produced by parts working individually, in pairs, in triplets, or in more complicated configurations.

Armed with this way of breaking down the arrow of time, the researchers analyzed existing experiments on the response of neurons in a salamander retina to different movies. In a movie, a single object moved randomly across the screen while another depicted all the intricacy of the scenes found in nature. In both films, the researchers found that the arrow of time emerged from simple interactions between pairs of neurons, not large, complicated groups. Surprisingly, the team also observed that the retina showed a stronger temporal arrow when observing random motion than a natural scene. Lynn said this latest finding raises questions about how our internal perception of the arrow of time aligns with the outside world.

“These findings may be of particular interest to neuroscience researchers,” Lynn said. “They could, for example, lead to answers about whether the arrow of time works differently in neuroatypical brains.”

“Chris’ local irreversibility decomposition, also known as the arrow of time, is an elegant general framework that can provide a new perspective for exploring many high-dimensional non-equilibrium systems,” said Professor David Schwab. of Physics and Biology at the Graduate Center and the principal investigator of the study.

Authors in order: Christopher W. Lynn, Ph.D, postdoctoral fellow, CUNY Graduate Center; Caroline M. Holmes, PhD student, Princeton; William Bialek, Ph.D, professor of physics, CUNY Graduate Center; and David J. Schwab, Ph.D., professor of physics and biology, CUNY Graduate Center

Funding sources: National Science Foundation, National Institutes of Health, James S McDonnell Foundation, Simons Foundation and Alfred P Sloan Foundation.

About the City University of New York Graduate Center

The CUNY Graduate Center is a leader in public higher education dedicated to enhancing the public good through pioneering research, thoughtful learning, and reasoned debate. The Graduate Center offers ambitious students nearly 50 doctoral and master’s programs of the highest caliber, taught by top faculty from across CUNY – the nation’s largest urban public university. Through its nearly 40 centers, institutes, initiatives, and the Center for Advanced Scientific Research, the Center for Graduate Studies influences public policy and discourse and shapes innovation. The Graduate Center’s extensive public programs make it a hotbed of culture and conversation.

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